I currently have a Core i5 setup with 4gb ram and a ATI 5850, running a 24" 1920x1200 monitor. Thinking about upgrading my card, which one would be best? I really think about changing to the nvidia camp for a change

Budget can be as high as 500 euro. Crossfire might be an option but I don't think my mainboard has two PCIe slots (not sure about that), but I also have only a 600watt PSU which might not be enough to power 2 cards.

I currently have a Core i5 setup with 4gb ram and a ATI 5850, running a 24" 1920x1200 monitor. Thinking about upgrading my card, which one would be best? I really think about changing to the nvidia camp for a change

If you can wait, see where the 6990 lands and look into going that route.

570 and 6970 are about even. There may be a few titles that favor one over the other. Considering the 570s are a little cheaper right now I'd recommend that. 580s are just too expensive to be a good value, but if you want the best possible single gpu performance that's your only solution.

If you can find a very cheap 5850 and compatible PSU, crossfire actually might be a better solution. In applications that scale well with crossfire, your framerate will likely be higher than a single 580.